Add @__KEY__ annotation to mangle string literals

[jpdutoit]

Here I add a new annotation @__KEY__ that when placed before a string literal causes it to be mangled like a property.

Example:

function lookup(object, key) {
   return object[key]
}
let result = lookup({ somekey: 5}, /*@__KEY__*/ "somekey") 
console.log(result) // prints 5 even if somekey was mangled

Do you think this should be part of Terser? Are you happy with the annotation name?

I'm leaving the readme and tests until it is decided that this is a useful feature...

[fabiosantoscode]

Looks like a good idea!

I'm interested in seeing a test for /*@__key__*/ 'mykey' in obj because from a cursory look in the diff, it looks like mangle() might be called on that left-hand string twice (once for the "in" operator's left side, and again for the string itself).

[jpdutoit]

I don't think it will double mangle, as the mangled string shouldn't be in the allow-list generated in the "find candidates to mangle" step, unless the mangled string is accidentally the same as one in that list 🤔

I will add a test as soon as I figure how how those mangle tests work...

[fabiosantoscode]

property mangle tests are located here:

https://github.com/terser/terser/blob/master/test/compress/mangleprops.js

You can run them with npm run test:compress -- mangleprops.js. They're basically input and output by example. options = assigns compress options and mangle = assigns mangle options. The input: {} and expect: {} labelled blocks are the input/expected output pairs.

Best of luck!

[jpdutoit]

I managed to construct a failing case. It only triggers if you have an unrelated property elsewhere with the same name as the mangled property. I will fix this by clearing the annotation whenever an AST_string is mangled.

[jpdutoit]

Ok, readme and tests are in!

[fabiosantoscode]

This is cool, thanks for the contribution!